This invention relates generally to radioactivity well logging and, more particularly, to methods and apparatus utilizing natural gamma ray well logging for evaluating subsurface earth formations.
Various models and apparatus have been utilized in the art of well logging to study the radioactive properties of subsurface formations, both where the radioactivity is natural and where it is artifically induced. In general, one such method and apparatus, when applied to the field of natural gamma ray well logging, has utilized channels of spectra including those of potassium-40, uranium and thorium. One such technique utilizes these natural gamma ray logs to determine characteristics of shale formations as described in U.S. Pat. No. 4,071,755, issued Jan. 31, 1978 to Supernaw et al. Unfortunately, it can be difficult to differentiate such shale formations from other formations having relatively high concentrations of uranium, such as uranium-rich sands or formations exhibiting uranium oxide plating as a result of fluid movement.
Thus, the present invention provides methods and apparatus for evaluating subsurface formations to differentiate between shale formations and other formations having relatively high concentrations of uranium.